WASHINGTON – The concern on Monday over Russian hacking of the presidential campaign became another indication that the election clashes between Sen. John McCain and President-elect Donald Trump could live on under the new administration.

McCain – in stark contrast to Trump – said Russia waged a type of warfare when it hacked the presidential election and he plans to call committee hearings to investigate following a bombshell CIA claim the Russians were attempting to benefit Trump.

“The entire issue is going to be examined by the Armed Services Committee because it is a threat to our national security,” McCain said about the Russian computer hacking during an interview with CBS.

Trump, who campaigned on a closer relationship with Russia, questioned whether that country was behind the hacking at all and called the CIA allegation a ridiculous excuse by Democrats to explain a surprise election loss.

“They have no idea if it’s Russia or China or somebody. It could be somebody sitting in a bed some place,” the president-elect said Sunday in a Fox News interview.

In the month since the presidential election, McCain has challenged the incoming Trump administration again and again on Russia, waterboarding and the defense budget. The Arizona Republican is emerging as a top Republican willing to publicly push back against the president-elect on a range of issues.

McCain lashed out during a security forum last month in response to Trump’s call for waterboarding of terrorism suspects – and potentially much more severe methods of interrogation.

“I don’t give a damn what the president of the United States wants to do. We will not waterboard,” McCain said.

The split also surfaced Friday in a budget debate.

Republicans in Congress decided to put off passing annual federal spending bills until the end of April to accommodate the Trump administration. McCain opposed the delay in a fiery floor speech, saying it would hold up $6 billion in needed funding for the military.

“What a sham, what a fraud,” McCain said. “Is there any wonder why the American people hold us in such contempt, why we’re down to paid staff and blood relatives?”

McCain won re-election in November and will return next year as the chairman of the Armed Services Committee, where he could wield the committee’s influence to push military and foreign policy priorities that differ with the Republican president, such as the Russian hacking investigation.

The divisions and bad blood stretch back to the beginning of the campaign trail in July 2015.

Trump questioned whether McCain was a war hero because he had been taken prisoner in Vietnam, and McCain withdrew his endorsement of Trump in October when recordings surfaced of the billionaire businessman bragging about groping women’s genitals.

But so far Russia has become the top sticking point between McCain and Trump.

Less than a week after the election, McCain issued a statement strongly condemning a Trump proposal for the closer relationship with Russian President Vladimir Putin – and a possible partnership with Russia and Syria President Bashar Assad in resolving that country’s more than five-year civil war.

“At the very least, the price of another ‘reset’ would be complicity in Putin and Assad’s butchery of the Syrian people,” McCain said in November. “That is an unacceptable price for a great nation. When America has been at its greatest, it is when we have stood on the side those fighting tyranny.”

McCain has been a hawk when it comes to Russia policy, holding numerous Armed Services Committee hearings on its growing assertiveness that he and many people in the Republican establishment feel are a top threat to the United States.

Trump praised Putin during the campaign and earlier this month again suggested a new partnership with Russia to fight terrorism. He also appeared poised to pick a secretary of state nominee, ExxonMobile CEO Rex Tillerson, who has worked closely with Putin and Russia to secure energy deals.

“Why not get along with Russia? And they can help us fight [the Islamic State group], which is both costly in lives and costly in money, and they’re effective and smart,” Trump said in a Time interview published Dec. 7.

On Monday, McCain got some support on his push to investigate the Russian election hacking from another powerful Republican, Senate Majority Leader Mitch McConnell, who said the Senate Intelligence Committee will also look into the release of thousands of stolen Democratic emails released through Wikileaks during the election.

Sen. Lindsey Graham, R-S.C., a Trump critic and longtime McCain ally, and two Senate Democrats including New Yorker Chuck Schumer, who will become the senate minority leader, had already joined McCain in raising the alarm over the hack.